Oil-based or oil emulsion cutting fluids have been used for machining and deformation processing of metals and other materials. These fluids serve to reduce friction and wear, to remove heat, and to flush out generated debris. The removal of heat from the cutting tool prevents ductile deformation from thermal softening and fatigue failure from repeated thermal stresses. The removal of heat from the workpiece surface enhances the surface integrity by eliminating/reducing the occurrence of microstructure bends, white etch layers, phase transformations, changes in composition, or work hardening from excessive plastic deformation. Controlling the heat generated during machining will also prevent surface cracks and will increase the probability of inducing preferred compressive residual stresses at the surface.
The removal of heat is critical for metals with low thermal conductivity such as titanium, and also for metals that are difficult to machine, such as some stainless steels, nickel superalloys, and titanium. Usually different cooling methods are used to deliver the coolant in the vicinity of cutting where the chips, tool edge, and workpiece are in contact. Unfortunately, even with the use of high pressure coolant it is still a challenge to be able to reduce the heat generated during cutting, especially in the close vicinity of the cut. To increase the cutting speed and tool life (in order to increase productivity and reduce cost) it is recommended to apply more than just the flood coolant in the cutting zone.
More recently, cryogenic machining using liquid nitrogen or carbon dioxide as the cutting fluid has been used to improve heat removal (i.e., cooling via a large temperature differential). Cryogenic machining dramatically increases carbide tool life and enables higher cutting speeds. Cryogenic liquids do not appear to provide significant lubricity. A graphite or MoS2 dry lubricant aerosol can also be co-fed to prevent tool attritious wear, but can lead to the possible generation of micro-level defects due to welding of small particulates on the finished surface. In addition, certain tool-workpiece material couples do not machine well under the “dry” cryogenic conditions. For example, the benefits for using cryogenic coolants have not been found to be significant for machining nickel-based materials.